Attitude control of a spin stabilized spacecraft has traditionally been accomplished by pulsed thrusters which provide spin phased moments. This works well when the operational spin axis is normal to the orbit plane. The payloads in some spacecraft, however, require a nadier pointing spin axis. In order to alleviate the necessity for continuous thrusting to maintain the orbital angular rate, momentum canceling wheels are hardmounted to current spacecraft to create a zero momentum system. When it is desired to slew the spin axis from one pointing direction to another, reaction control thrusters are used to create the starting and stopping moments. If solar radiation pressure creates undesired external moments, these thrusters are also used to cancel such moments.
While reaction control thrusters are necessary, it would be desirable to use them as little as possible. There is only a limited amount of propellant available to fire the thrusters and the more that the thrusters must be fired, the less fuel is available for later maneuvers. Further, there are inherent uncertainties involving firing a thruster on time and sophisticated methods are required in order to compensate for these uncertainties.
U.S. Pat. No. 5,441,222 to Rosen discloses an attitude control system that is intended to minimize the use of thrusters. The attitude control system utilizes an agile gimbaled momentum wheel to provide attitude control moments and cancel the momentum of a spinning satellite and thus provide a zero overall system momentum. The disclosed momentum wheel assembly executes a series of coning motions to provide the correct control moments. The coning motion necessary for body attitude control and momentum control tends to require significantly more angular motion than that needed for wobble control. This is especially true when balancing techniques are employed (either passive or active) which can reduce the spacecraft wobble to an arbitrarily small value. With this configuration, the momentum wheel gimbal actuators typically need to provide large excursion, high rate, long life, and low disturbance torque position control. This results in a complex and costly implementation, such as disclosed in U.S. Pat. No. 5,820,078 to Harrell.